Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the wellbore, along with data relating to the size and configuration of the borehole itself. The collection of information relating to conditions downhole is commonly referred to as “logging.”
In conventional oil well wireline logging, a probe or “sonde” housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The upper end of the sonde is attached to a conductive wireline that suspends the sonde in the borehole. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
Commonly, a single transformer is used uphole to couple downlink signals onto the wireline and remove (i.e. strip) uplink signals from the wireline. Likewise, a single transformer may be used downhole to couple uplink signals onto the wireline and remove downlink signals from the wireline. More specifically, the uphole (surface) transformer acts to couple signals from the uphole transmitter to the cable and remove signals on the cable from the downhole transmitter for processing by the uphole receiver. The downhole transformer typically performs the reverse operation, i.e., the downhole transformer acts to couple signals from the downhole transmitter to the cable and remove signals on the cable from the uphole transmitter for processing by the downhole receiver. Signal outputs from these transformers are fed into frequency filtering networks to separate uplink signals from downlink signals both at the surface and downhole. The goal of the filtering circuits is to remove the strong local signal so that it does not interfere with the weaker signal coming from the opposite end of the cable.
Unfortunately, frequency filtering reduces the total amount of available bandwidth that may be used for telemetry signals. Additionally, it is difficult to attain optimal signal transmission and reception with a single coupling transformer since uplink and downlink signals are typically transferred over separate frequency bands (i.e., the transformer itself may filter one frequency band more than another). A telemetry system that overcomes these issues would be desirable. In general, methods and systems that increase the data rate capacity and/or reliability of uplink and downlink data transmissions are continually being sought.